Site Description Penrhyn Quarry makes up the largest slate workings in the world, and has remained in continuous production since long before the capitalisation of the workings by the first Lord Penrhyn in the late eighteenth century. By 1898 the quarry employed 2809 men, and 1916 men in 1937-8.
The quarry was worked partly as a stepped gallery and partly as a pit, but from about 1850 it began to employ water-balance shafts (e.g. NPRNs 33616, 400729) to raise loaded wagons. The Italianate-style quarry office (NPRN 308403) was built around 1860 during a period of expansion. Production of roofing slates remained entirely unmechanised until the introduction of electricity in 1912 led to the building of a range of mills on 'Red Lion', the main quarry level, and the installation of 'blondin' ropeways in the upper quarry. In the mid-1960s the site was bought by McAlpine which led to extensive modernisation.

In the nineteenth century Penrhyn, along with Dinorwic Quarry, dominated the Welsh slate industry. Penrhyn Quarry holds a significant place in the history of the British Labour Movement as the site of two prolonged strikes by workers demanding better pay and safer conditions. The first strike lasted for eleven months in 1896, and the second began in November 1900 and lasted for three years.